The production of higher hydrocarbons by the catalyzed reaction of CO and hydrogen is known familiarly as the Fischer-Tropsch reaction. Commercial plants have operated in Germany, South Africa and other parts of the world based on the use of particular catalysts. The German commercial operation, for example, concentrated on the use of a precipitated cobalt-thoriakieselguhr fixed-bed catalyst, and a later modification where MgO, for economy reasons, replaced part of the thoria.
Nickel is believed to be the original, and thus perhaps the best known, catalyst for the conversion of CO and hydrogen to methane (Sabatier and Senderens -- Compt. Rend. 134, 514 (1902); Catalysis Reviews 8, 159 (1973), G. A. Mills and F. W. Steffgen; and Catalysis, Vol. IV, P. H. Emmet, editor -- Reinhold Publishing Corp., 1956, p. 109). Cobalt catalysts were later found to be preferred for the synthesis of higher hydrocarbons over the use of nickel since less methane was produced (see "Conversion of Petroleum", Sachanen, 2nd Ed., p. 174). Iron containing catalysts have also been used commercially, for example, in South Africa; but while a spectrum of hydrocarbons are produced, CO.sub.2 is also produced as a byproduct, and this is undesirable. (See Paraffins-Chemistry and Technology, by R. Asinger, Pergaman Press, 1968, p. 90 et seq.) Ruthenium is also known as a Fischer-Tropsch catalyst. Ruthenium produces solely methane by the reaction of CO + H.sub.2 at atmospheric pressure (see Catalysis Review, 8, 159 (1973), G. A. Mills and F. W. Steffgen). But ruthenium is also known to surpass all other catalysts in the production of high melting waxes at high pressures of 50 to 1000 atmospheres (see Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 4, p. 447).
It would thus appear from the prior art that (1) iron catalysts have the disadvantage in Fischer-Tropsch synthesis of producing unwanted CO.sub.2 ; (2) nickel catalysts tend to produce too much methane; (3) cobalt catalysts are preferred for the production of a spectrum of hydrocarbons while avoiding the production of CO.sub.2 ; and (4) ruthenium catalysts produce only methane at low pressure and high melting waxes at very high pressures.
It is also known (see Catalysis, Vol. IV, P. H. Emmet, editor -- Reinhold Publishing Corp., 1956, pp. 29-31) that a nickel doped cobalt catalyst increases the methane content of the product over the use of cobalt alone since each component of the catalyst, i.e. nickel and cobalt, tend, as expected, to retain their own peculiar characteristics when the components are combined into a single catalyst. Similarly, U.S. Pat. No. 3,787,468 to Donald Kingsley Fleming et al, entitled "Methanation of Carbon Monoxide and Carbon Dioxide", teaches the addition of ruthenium to a known rhodium or rhodium-platinum-tungsten oxide catalyst for the production of methane from the reaction of CO and hydrogen, or CO.sub.2 and hydrogen mixtures, at temperatures in the range of 75.degree. to 250.degree. C. Thus Fleming et al teach the addition of ruthenium to a certain type of mixed metal catalyst does tend to selectively produce methane as shown in FIG. 2 of the patent.
It has now been found in accordance with the invention, and contrary to expectations, that the addition of small amounts of ruthenium to a catalyst containing a major amount of cobalt as the active ingredient in a hydrocarbon synthesis catalyst results in the substantial elimination of methane in the product (rather than an increase in methane) in a low pressure synthesis gas process operated under normal synthesis process reaction temperatures with the simultaneous shift to the production of a higher carbon number product having a lower olefin content.
The surprising feature of the use of a ruthenium promoted cobalt catalyst in the synthesis of hydrocarbons from the reaction of CO and hydrogen is that the benefits of the use of ruthenium which were only achieved previously at very high pressure are now available at substantially atmospheric pressure. Stated another way, it was expected that the synthesis gas process would yield increased amounts of methane in the product when operated at low pressures in the presence of a cobalt catalyst containing small amounts of ruthenium. For reasons not understood, the addition of small amounts of ruthenium to a cobalt synthesis catalyst resulted in the substantial elimination of methane from the product, together with the other benefits noted above, in the production of a more saturated, higher average carbon number product.